Experience with stacked subwoofers?

@wokeuptobose 

I have been dialing in my second pair of REL S/510 as a four-pack array, while I wait for delivery of the third pair. Even going from a stereo pair to a four-pack has been an amazing revelation. My wife was skeptical, but she is enjoying the change immensely and starting to ask me to play stuff she loves to see how much better it sounds. She's now asking when the last pair will arrive and get integrated.

I should get my third pair of S/510 soon to complete the six-pack. This is becoming a revolutionary change to my system. MIND. BLOWN.

@wokeuptobose & @dwette 

YES, Yes, yes.  This is what I heard at the AXPONA demo.  While the six-pack clearly delivers more bass (yeah, they're subwoofers, so that's obvious) the impact of the RELs was shockingly apparent with regard to: (1) size of the soundstage (2) precision of imaging.

I went into the demo thinking this was just RELs attempt to sell more subs (a "marketing gimmick" as @emergingsoul put it) or more charitably an attempt to even out bass performance (as multiple members have implied with their comments).  I left the room shocked and awed. My first thought was... how can I ever again assess the sound quality of ANY pair of speakers without an accompanying REL six-pack?  I'm sure many will write me off as a kook.  That's fine with me.  I heard what I heard. 

Which manufacturers other than Rel make subwoofers specifically designed to be stacked?  I’m assuming here that there are different design decisions that a manufacturer would need to make for their subs to be optimized for stacking.  In particular, are there any stackable subs below the Rel 510 price point?

These guys are marketing a change in the sound dynamics in a room. Exploiting human nature.  Just because you hear a change doesn't mean things are getting better.  So now what are we gonna start doing stacked mid range and tweeters??

All this Because main speakers are now this made with smaller diameter bass drivers.

"I'm sure many will write me off as a kook."

"These guys are marketing a change in the sound dynamics in a room. Exploiting human nature.  Just because you hear a change doesn't mean things are getting better."

As anticipated. Talk amongst yourselves...